Heat the oil in a large pot over medium heat. Add the onion and celery and cook until softened, about 5-7 minutes.

Add the celeriac and potatoes and cook for another 5 minutes, stirring occasionally.

Add the garlic and cook for 1 minute more.

Pour in the vegetable broth and bring to a boil. Reduce heat and simmer for 20-25 minutes, or until the vegetables are tender.

If using, add the spinach during the last 5 minutes of cooking.

Carefully blend the soup with an immersion blender or in a regular blender until smooth. Season with salt and pepper to taste.

Serve hot.